It is well known that tertiary recovery from an oil field can be enhanced by down-hole injection of nitrogen, carbon dioxide and steam, individually or in selected mixtures, at controlled temperatures and pressures. Such injection may be continuous at an injected well with recovery had at a production well spaced therefrom, or cyclic with recovery at the injected well by the so-called "huff and puff" method. The requirements, as respects such gaseous components and mixtures, as well as temperatures and pressures, vary from field to field. Temperature requirements may vary from a low of 60.degree. F. for miscible flooding of light crudes to a high of 1200.degree. F. for very heavy crudes, but in practice injection temperatures are limited to that which well casing will withstand, i.e., of the order of 1,000.degree. F. Similarly, pressure requirements can be quite high, i.e., up to 8,000 psi, but injection pressures are limited by practical considerations, i.e., the pressure capability of the apparatus employed to generate the gaseous components.
Publications relating to such recovery enhancement are:
"Engine Exhaust Gas Boosts Heavy Oil Recovery", by Norman J. Clark et al, Petroleum Management, August 1964 PA0 "Miscible Displacement With Nitrogen", by Michael D. Rushing et al, Petroleum Management, November 1977 PA0 "Nitrogen May Be Used For Miscible Displacement in Oil Reservoirs", by Michael D. Rushing et al, JPT Forum--Paper Presented at the SPEAIME Deep Drilling and Production Symposium, Amarillo, Tex., Apr. 17-19, 1977 PA0 "Enhanced-Recovery-Inert Gas Processes Compared", by Keith Wilson, The Oil and Gas Journal, July 3, 1978 PA0 "Nitrogen-Driven Co.sub.2 Slugs Reduce Costs", by J. P. O'Leary et al, Petroleum Engineer International, May 1979.
Various types of apparatus have been proposed, and even constructed and used, for down-hole injection of the aforementioned gaseous components. Among the earliest of such proposals is that disclosed in the patent to Day U.S. Pat. No. 1,342,741, June 8, 1920. Day proposed the above-ground fixed installation of a large high-pressure gas generator or combustion reactor for the projection thereinto, under high pressures, and burning therein of a combustible mixture of oil, or oil and steam, and air, to generate hot pressurized combustion gases for injection down-hole to enhance recovery from bituminous shale or rocks.
The patent to Hixon U.S. Pat. No. 2,173,556, Sept. 19, 1939 and to Baldwin U.S. Pat. No. 3,066,737, Dec. 4, 1962, disclose portable apparatus comparable to that disclosed in Day for the production of combustion gases for down-hole injection at high temperatures and pressures.
Problems arise, however, with apparatus of the type disclosed by Day and Hixon. In particular, combustion under pressure raises flame temperature. At the higher injection pressures contemplated here, e.g., over 1500 psi, flame temperatures may reach 5,000.degree. F. or more. No practical refractories are available which will withstand such temperatures.
The patent to Walter U.S. Pat. No. 2,734,578, Feb. 14, 1956, sought to overcome the high flame temperature problem with a cooling water jacket, which also was used to produce steam, but that only created another problem, i.e. the formation of soot on cooled metal surfaces which is entrained with combustion gases and can readily clog subterranean formation pores, thus greatly inhibiting the benefits otherwise achievable. Accordingly, Walter employed downstream scrubbers to remove the soot before down-hole injection. Walter further discloses, however, various modifications of his apparatus for producing only steam and carbon dioxide (CO.sub.2) or nitrogen (N.sub.2) and steam for down-hole injection. Walter's combustion reactor used for such selective production, however, differs from and is not interchangeable with that used to produce all three gaseous components, i.e., N.sub.2, CO.sub.2 and steam. Walter had the further idea, disclosed in his later U.S. Pat. No. 2,839,141, June 17, 1958, of injecting combustion gases together with steam and free oxygen for effecting subterranean in-situ combustion. The steam was produced, however, by a separate boiler heated by a burner or by the exhaust from a prime mover.
Control of flame temperature by means other than a water cooling jacket also is known. The patent to Lange U.S. Pat. No. 3,700,035, Oct. 24, 1972, discloses apparatus for effecting in-situ combustion in subterranean oil fields, like Walter '141, by down-hole injection of combustion gases, steam and oxygen. The combustion gases are generated in a small-volume, high-pressure, water-jacketed combustion reactor fed with oxygen-enriched fuel which generates extremely high flame temperatures. In Lange the flame temperature is reduced and controlled by injecting water directly into the flame thereby reducing its temperature by evaporation. The Lange water injection method, however, could form soot where the water contacted the flame prior to complete combustion. Further, it would seem that the Lange water-jacket also results in soot formation on its inner metal surfaces exposed to the flame, the same as in Walter '578, because Lange uses a bed of small refractory bodies at the outlet of the combustion chamber to filter out entrained solids.
The patents to Hamrich et al U.S. Pat. No. 4,077,469, Mar. 7, 1978, and Rose et al, U.S. Pat. No. 4,159,743, July 3, 1979, both disclose down-hole burners for the generation of combustion gases to enhance recovery from an oil field. The combustion reactors are water-jacketed and in their combustion zones are lined with refractory material. Beyond that zone, however, the metal walls of the jackets are exposed and perforated for the injection of water to cool the hot combustion gases. Again it would seem that objectionable soot would be formed on the exposed parts of the metal walls of the water jackets. Further, down-hole gas generators are necessarily of such small volume that the generation of combustion gases at the required rates cannot be attained.
Present practice usually involves purchase of CO.sub.2 and N and to heat and compress such gases at the injection site, a very costly process.